Task demand mediates the interaction of spatial and temporal attention.

Psychophysical studies typically test attentional mechanisms in isolation, but in everyday life they interact to optimize human behavior. We investigated whether spatial and temporal attention interact in two orientation discrimination experiments that vary in task demand. We manipulated temporal and spatial attention separately and conjointly with well-established methods for testing each spatial or temporal attention. We assessed sensitivity (d') and reaction time for every combination of spatial and timing cues, each of which was valid, neutral, or invalid. Spatial attention modulated sensitivity (d') and speed (reaction time) across temporal attention conditions. Temporal attention modulated sensitivity and speed under high- but not low- task demands. Furthermore, spatial and temporal attention interacted for the high-demand task. This study reveals that task demand matters; in a simple task spatial attention suffices to improve performance, whereas in a more demanding task both spatial and temporal attention interact to boost performance, albeit in a subadditive fashion.

Microsaccades and temporal attention at different locations of the visual field.

Temporal attention, the prioritization of information at specific points in time, improves performance in behavioral tasks but cannot ameliorate the perceptual asymmetries that exist across the visual field. That is, even after attentional deployment, performance is better along the horizontal than vertical meridian and worse at the upper than lower vertical meridian. Here we asked whether and how microsaccades-tiny fixational eye-movements-could mirror or alternatively attempt to compensate for these performance asymmetries by assessing temporal profiles and direction of microsaccades as a function of visual field location. Observers were asked to report the orientation of one of two targets presented at different time points, in one of three blocked locations (fovea, right horizontal meridian, upper vertical meridian). We found the following: (1) Microsaccade occurrence did not affect either task performance or the magnitude of the temporal attention effect. (2) Temporal attention modulated the microsaccade temporal profiles, and this modulation varied with polar angle location. At all locations, microsaccade rates were significantly more suppressed in anticipation of the target when temporally cued than in the neutral condition. Moreover, microsaccade rates were more suppressed during target presentation in the fovea than in the right horizontal meridian. (3) Across locations and attention conditions, there was a pronounced bias toward the upper hemifield. Overall, these results reveal that temporal attention benefits performance similarly around the visual field, microsaccade suppression is more pronounced for attention than expectation (neutral trials) across locations, and the directional bias toward the upper hemifield could reflect an attempt to compensate for typical poor performance at the upper vertical meridian.

Cadmium, copper and lead levels in different cultivars of lettuce and soil from urban agriculture.

Urban agriculture plays an important role in sustainable food supply. However, because of the atmospheric pollution and soil contamination associated with urban areas, this activity may be of concern. In fact, contamination of soil with metals and the transference of contaminants to vegetables can represent health and safety risks associated with urban agriculture. The objective of this study was to evaluate the concentrations of selected trace metals (cadmium, copper and lead) in three lettuce cultivars produced in three different urban gardens in the metropolitan region of Belo Horizonte, Brazil and their respective soils. Samples of lettuce and soil were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) and graphite furnace atomic absorption spectrometry (AAS-GF), respectively, and their transfer coefficients were calculated. The methods were optimized and were fit for the purpose. Copper was the prevalent metal found in soils and lettuce, with an average of 27.9 ±â€¯13.9 and 0.608 ±â€¯0.157 mg kg-1 respectively, followed by lead (19.4 ±â€¯7.7 and 0.037 ±â€¯0.039 mg kg-1), and cadmium (0.16 ±â€¯0.03 and 0.009 ±â€¯0.005 mg kg-1). Cadmium presented the largest transfer coefficients, ranging from 0.34 to 1.84 with an average of 0.92 ±â€¯0.45, which may indicate a potential risk of accumulation in vegetables in the case of high soil contamination. A significant positive correlation was observed (p < 0.01) between cadmium in lettuce and in soil. Even though lead concentrations varied in the soils from the different urban areas, ranging from 11.88 to 30.01 mg kg-1, no significant difference (p < 0.05) was found among the lettuce, probably due to its low mobility (transfer coefficient = 0.02). The copper and cadmium levels found in lettuce indicate safe lettuce production in the three urban gardens.

Speciation, distribution, and transport of mercury in contaminated soils from Descoberto, Minas Gerais, Brazil.

This work describes an investigation of mercury contamination in an abandoned gold mining site in the rural area of Descoberto, Minas Gerais, Brazil, whose inhabitants have reported "silver balls" present in the soil. Different granulometric fractions of soil samples and sedimented material from rainwater retention tanks in this area were analyzed for total mercury, organic matter, and mercury speciation by thermodesorption/atomic absorption spectrometry. The results showed mercury concentrations in the soils in the range of (0.0371-161) mg kg(-1), and the occurrence of Hg(0) oxidation. Some samples had concentrations as high as 90 mg kg(-1) with the majority as Hg(2+), which is important information in order to understand the biogeochemical behavior of mercury in contaminated sites and to apply the appropriate remediation technology. The retention boxes and tank samples showed that fine particles with high mercury content (3.3-90) mg kg(-1) are leached from the contaminated area, which reveals the need for efficient control of this material to prevent the contamination of stream waters. This study is an example that may be useful for other contaminated sites.

Quantification and speciation of mercury in soils from the Tripuí Ecological Station, Minas Gerais, Brazil.

Contents of total mercury, organic carbon, total sulfur, iron, aluminum and grain size and clay mineralogy were used along with Pearson's correlation and Hg thermal desorption technique to investigate the presence, distribution and binding behavior of Hg in soils from three depths from the Tripuí Ecological Station, located near Ouro Preto, Minas Gerais State, Brazil. The soils studied had predominantly medium and fine sand texture (0.59-0.062 mm), acid character and Hg contents ranging from 0.09 to 1.23 microg/g. The granulometric distribution revealed that Hg is associated with coarse sand (2-0.59 mm) and silt and clay (<0.062 mm) and presents similar Hg concentrations in both fractions. Mercury distribution in soil profiles showed that Hg was homogeneously distributed throughout the depths at most sites. Hg thermal desorption curves show that mercury occurs not only as Hg2+ predominantly bound to organic components in most of the samples, but also in the form of cinnabar in some. Pearson's correlation confirmed that mercury is associated with organic matter and sulfur and possibly with sulfur-bearing organic matter in most samples.